1. Field of the Invention
This invention relates to an electric resistance alloy consisting essentially of palladium and iron with a small amount of impurities, which alloy is stable at very high temperatures. More particularly, the invention relates to an alloy material for electric resistive elements having a small change of electric resistance over a wide temperature range of 490.degree.-1340.degree. C. and yet being easily workable at room temperature by forging, rolling, drawing, winding, shaping, and the like.
2. Description of the Prior Art
The need for measurement at high temperatures under very stringent conditions has been increasing these years in various industries, such as iron manufacturing industry, chemical industry, nuclear industry, space-related industry, and the like.
For instance, in the continuous casting process, the top surface of molten metal in a tundish or mold must be continuously controlled at a desired level, so as to ensure continuous production of iron or steel goods of high quality with a high yield through an uninterrupted casting process. Conventional level meters for molten metal which use .gamma.-ray, X-ray, or other radioactive ray, have a shortcoming in that they are bulky and have safety problem. To overcome this shortcoming, the use of an eddy-current type displacement meter (to be referred to as "the displacement meter" hereinafter) of small size has been contemplated recently.
The performance of the displacement meter depends on the material of sensor coils assembled therein, so that the properties of the sensor coil material, such as electric characteristics, response to ambient conditions during use, and stability, are very important. For example, in the case of the continuous casting, the temperature of the molten metal can be as high as 1,500.degree. C., and the sensor coils which are located immediately above the molten metal are required not only to withstand high temperatures of about 1,000.degree. C. but also to maintain their utmost performance with a high stability over a long period of time as essential quality thereof.
The inventors disclosed a palladium-silver alloy (to be referred to as "the Pd-Ag alloy" hereinafter) consisting essentially of 55.5 to 60.6 wt.% of palladium and 44.5 to 39.4 wt.% of silver for the sensor coils of the displacement meter for use at high temperatures (see Japanese Patent Laying-open Publication No. 122,839/80). The Pd-Ag alloy has excellent corrosion-resistances and acid-resistances and good workability at high temperatures, and furthermore, the alloy is characterized by its very small temperature coefficient of electric resistance of less than +20 ppm/.degree.C. over a wide temperature range of -50.degree. C. to +600.degree. C. (as shown by a curve for the reference alloy in FIG. 1). However, at the very high temperatures of 600.degree.-1,000.degree. C., the Pd-Ag alloy shows a large temperature coefficient of electric resistance of +133 ppm/.degree.C., so that the sensor coils made of the Pd-Ag alloy are susceptible to large drifts at the very high temperatures such as those experienced in the above-mentioned continuous molding, and the accuracy of the displacement meter using such sensor coils is rapidly reduced at such very high temperatures and accurate measurement of level cannot be ensured. Accordingly, there has been a pressing need in various industries for novel material of sensor coils which ensures high accuracy of measurement in a very stable fashion at the very high temperatures in excess of 600.degree. C.